To debug a custom workflow activity, copy the .pdb file for the assembly to the %installdir%\server\bin\assembly folder. The assembly can be deployed as on-disk or stored in the database. The recommended deployment is in the database, but for debugging you should select on-disk. Next, attach the debugger to the CrmAsyncService.exe process. Make sure that you remove the .pdb file when you’ve finished debugging because it uses memory to have it loaded. For detailed information, see Debug a plug-In.